Abstract: A photobioreactor system for efficient oxygen production for a closed ecological life support system (CELSS) is disclosed. Special features of this system include, e.g., the optical transmission system, uniform light distribution, continuous cycling of cells, gravity independent gas-exchange, and an ultrafiltration unit. The fiber optic based optical transmission system illuminates the reactor internally and includes a light source which is external to the reactor, preventing heat generation problems. Uniform light distribution is achieved throughout the reactor without interfering with the turbulent regime inside. The ultrafiltration unit exchanges spent with fresh media and its use results in very high cell densities, up to 10.sup.9 cells/ml for Chlorella vulgaris. The prototype photobioreactor system may be operated in a batch and continuous mode for prolonged periods of time. The photobioreactor may be used to convert CO.sub.2 to oxygen in an artificial lung.